1. Field of the Invention
This invention relates to pressure sensitive adhesive compositions which include an amine terminated polyalkylene oxide neutralized sulfonated thermoplastic polymer, wherein the neutralized sulfonated thermoplastic polymer has about 5 to about 250 milliequivalents (meq.) of neutralized sulfonate groups per 100 grams of the neutralized sulfonated thermoplastic polymer, dissolved in a nonreactive solvent at a concentration level of about 5 grams to about 25 grams per 100 ml. of non-reactive solvent. To control the end use properties of the solvent-based pressure-sensitive formulations, optionally, tackifying resins can also be incorporated in the solution.
Broadly speaking, synthetic adhesives used in packaging can be classified into four categories: water based, solvent based, reactive and hot melt adhesives. Of these four, currently the water based are used most extensively. Usually the water based adhesives are based on emulsion polymers and are applied to porous cellulosic substrates. Energy from the outside in some fashion is applied to the system to evaporate the water in order that a strong bond may be formed. Besides this energy requirement for the formation of the bond, there is another complication with the use of water-based adhesive. For a uniform coating, a good unform wetting of the substrate surface is desired, which is not easily achieved.
The key requirements for pressure-sensitive adhesives are that they should have good cohesive and tackifying properties at ambient conditions. These adhesives should also have good flow characteristics if they are to be used in the bulk state, otherwise they should possess sufficient consistency when dissolved in suitable solvents so that they can be coated or sprayed on the surfaces to be bonded.
2. Prior Art
Several U.S. Patents have described sulfonated polymers such as sulfonated Butyl rubbers and sulfonated EPDM in adhesive applications (e.g., U.S. Pat. Nos. 3,867,247 and 3,801,531). It is important to distinguish the instant invention over those prior art systems. The former patent is directed at a sulfonated butyl cement which is solvent-based and is employed to laminate various substrates. It is important to note that the instant invention differs dramatically from that patent as follows:
(a) The instant invention may optionally include a tackifier resin;
(b) The instant invention is directed to sulfonated thermoplastic polymers which are neutralized with amine terminated polyalkylene oxides; and
(c) The instant invention is directed at sulfonated thermoplastic polymers whereas most of the prior art deals with sulfonated Butyl rubber (e.g., U.S. Pat. No. 3,867,247).
With regard to the latter point, historically polystyrene systems do not possess tack properties. This problem becomes even more acute when polystyrene is sulfonated to levels of 5 to 250 meq. per 100 grams of polymer and neutralized. The resulting compositions have been widely used as the basis for ionically crosslinked plastics. The use of these materials in such applications is, in part, a demonstration that the properties of such materials are just the opposite of those desired for adhesive. In fact, such materials are remarkably devoid of tack or adhesion. The art, therefore, of converting such physically crosslinked materials into suitable adhesive candidates is a particularly challenging one for two reasons: (a) the polystyrene backbone is particularly unsuited for that purpose being a very dry thermoplastic; and (b) the strong associations attributable to metal sulfonate crosslinks further inhibit adhesion to any particular substrate.
Despite these handicaps, there are some very good reasons for solving the problems associated with converting sulfonated polystyrene into a good pressure sensitive adhesive composition. The excellent thermal stability inherent in the polystyrene backbone is a very desirable property for adhesives, which will be exposed to high temperatures for long times. Most adhesives based on other elastomeric backbones can suffer degradation under those conditions.